I can't speak for everyone, but I will tell you why it is a monstrously big deal for our family. While it doesn't matter as much in grade school if you are in a school where the teachers are good at instructional differentiation (and that's a big IF), it starts to matter a whole lot in middle school. And staying tracked in gifted in middle school is often dependent upon being identified in grade school. I'll divide my argument into two prongs: a. math, and b. humanities.
Preliminarily, it is way easier to get into the Hopkins CTY program and the UVA summer gifted programs if you are pre-identified by your schools, but that's just a side note. Nevertheless, a way to STAY advanced is to take those Hopkins math and science courses over the summer, which you have to test in for, but I digress.
Anyway, from my point of view, here is why it really matters in middle school: a. for math: unless your kid is identified as in the gifted/AAP or whatever trendy label, in middle school, then they will not get to take the advanced math classes. Beginning in 6th grade, our school had 2 math tracks, regular and advanced. In 7th grade there are 3 math tracks, regular, advanced, and excellerated. AND, if your kid does not take advanced or excellerated math, then they most likely will not get to be in the advanced math classes in 8th and 9th grade. Here is the key to my view: UNLESS you have those advanced classes by 9th grade, then there are not enough semesters left in high school to complete, satisfy pre-requisites, and finish the highest and next to highest calculs courses by the time the kid graduates 12th grade. At least that's how it was in my DD's high school And, to make things more mysterious for parents, the middle school teachers do not normally know this and give bad advice. For math, it is not ONLY about making sure the student will be challenged and do well. It is really about making sure that they start high school at an advanced enough level to achieve the highest levels of math courses. Lots of students do not care about this when they get to high school, but why close off those options in middle school when the student doesn't yet know what their real goals will be in high school math? Also, not blaming middle school teachers since there is no way they can be clued into this, especially when they have been teaching for only a couple of years; they have enough to worry about.
And, regarding b., the humanities: except in the instances where the student is going to a high school where they can override the recommendations of the pre-requisite classes by teachers (these schools are rare but do exist), UNLESS the student in in advanced language arts, they will have a very hard time getting into the honors english and language arts and history classes, but it is not as critical as the math situation because often there WILL be time to make up the semesters to get to the highest class that they might want. So, for humanities, there ARE enough semesters to catch up usually even if you don't start out in the advanced track, unlike for math.
Okay, this is not important for most students, but for certain students, this is all really important to them; but you won't know the extent of the importance until the individual gets to high school. That's why these parents fight to keep the kid's options open.